1. Field of the Invention
The present invention relates to a control parameter setting method for a control circuit in a measurement control system and a measuring instrument.
2. Description of Related Art
A gain, phase compensation frequency, etc. are known as a control parameter of a control circuit. As a gain setting method for the control circuit in the measurement control system, there has been conventionally known a method including a data obtaining step for obtaining data required for gain setting, a determination step for determining an optimal gain for measurement based on the data, and a setting step for setting a gain determined to be the optimal gain in the control circuit (see, for instance, Document 1: JP-A-5-223519). Herein, the optimal gain of the control circuit is a gain capable of preventing the measurement control system from being in a self-oscillation state to achieve stable measurement as well as providing quick measurement.
For determination of the optimal gain, various methods can be used. For instance, methods (a) and (b) described below are disclosed in Document 1.
(a) A data group (material of a stylus, material of a workpiece, etc) required for the gain setting is manually input by an operator, which is then compared with a data table having been prepared in advance. A plurality of reference input data groups, reference gain values (proportional action parameter PP, etc.) obtained empirically as optimal gains respectively for the reference input data groups and the like are included in the data table. The operator compares an actual input data group input by oneself and the plurality of reference input data groups and selects a reference input data group most approximate to the actual input data group to set a reference gain corresponding to the selected reference input data group in the control circuit as the optimal gain.
(b) A stylus is fixed on a point of a workpiece and a temporary measurement is conducted while changing gains (proportional action parameter PP, etc.) of the control circuit to calculate a current error ΔE for each of the gains. ΔE represents a degree of dispersion of tunneling current applied between the stylus and the workpiece relative to a target current. In a measurement control state, since the tunneling current is changed in a manner oscillating around the target current, a large ΔE indicates that amplitude around the target current is large and the measurement control system is in a self-oscillation state (i.e., hunting is being generated), while a small ΔE indicates that amplitude around the target current is small and the tunneling current is substantially equalized with the target current, so that the measuring control system is stable. A determination of the optimal gain of the control circuit is performed for stabilizing the measurement control system, and a gain realizing small ΔE is determined to be the optimal gain. Specifically, a ΔE value for each of the gains is compared with a predetermined reference value, and a gain with the ΔE value equal to or smaller than the reference value is determined to be the optimal gain.
However, the methods (a) and (b) described above each have problems as follows.
In the method (a), since a preparatory measurement for preparing the data table has to be conducted precisely, a burden on the operator is increased. In addition, since the reference gain corresponding to the reference input data that is most approximate to the actual input data group is set as the optimal gain, the optimal gain is also a mere approximate value. Thus, in ultraprecision measuring or the like, where high precision is required in setting the gain, the method cannot be employed.
In the method (b), the optimal gain of the control circuit is determined with the ΔE value as an index to prevent the hunting from being generated in the measurement control system. However, the ΔE value is mere a value indicating the degree of dispersion of the tunneling current, which cannot be directly related with existence of the hunting. Thus, the small ΔE value does not always indicate that the hunting is not generated, so that the hunting might not be prevented sufficiently even by setting the optimal gain with the method (b).
Further, frequency characteristics of the entire measurement control system are changed depending on properties such as hardness and elasticity of the measuring unit and the workpiece.
Thus, even if the frequency characteristics of the measurement control system are maximized by setting the optimal gain in the control circuit by the gain setting method disclosed in Document 1 described above, when at least one of the measuring unit and the workpiece is replaced with another one having different properties, the frequency characteristics of the measurement control system are degraded and decreased, thereby possibly impeding stability of the measurement.